Tightening a bolt is really about stretching it: the stretch creates the clamp force, or preload, that holds a joint together and stops it loosening. But you can't measure stretch with a wrench, so we tighten to a torque instead and trust that it produces the preload we want. The link between the two is the simple rule T = K·F·d — and the weak point is K, the nut factor, which lumps all the friction into one number that lubrication and surface condition can swing dramatically.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: standard short-form torque-tension (T = K·F·d) practice.
The torque–preload equation
With preload F in kilonewtons and diameter d in millimetres, T comes out directly in newton-metres — convenient for metric bolts. The nut factor K is dimensionless and dominated by friction: around 0.2 for plain steel as received, lower when lubricated, higher when dry or corroded. Because preload is set only through K, two identically torqued bolts in different conditions can end up with very different clamp forces.
Worked example — an M12 8.8 flange bolt
Scenario: An M12 grade 8.8 bolt (tensile stress area 84.3 mm², proof strength 580 MPa) tightened to 70% of proof, as-received (K = 0.2).
So about 82 N·m gives a ~34 kN clamp. Lubricate the same bolt (K ≈ 0.15) and that 82 N·m would over-stretch it toward 45 kN — past proof — which is why lubricated joints must be torqued lower. Conversely a dry, slightly rusty bolt (K ≈ 0.3) torqued to 82 N·m would only reach ~23 kN, leaving the joint under-clamped and prone to loosening.
Frequently Asked Questions
T = K·F·d: nut factor × preload × diameter. With F in kN and d in mm, T is in N·m. Most torque overcomes friction, not stretch.
A friction coefficient for the whole joint: ~0.20 as-received, ~0.15 lubricated, ~0.30 dry/plain. It controls how torque maps to preload.
Commonly ~70% of proof load (proof strength × stress area). For M12 8.8 that's ~34 kN. Critical joints may go 75–90% with controlled methods.
Lube lowers K, so the same torque gives more preload — a dry torque on a lubed bolt can snap it. Match torque to the bolt's actual condition.
~45–50 N·m at K=0.2 and ~70% proof (≈24 kN). M8 ≈ 24 N·m, M12 ≈ 82 N·m, M16 ≈ 200 N·m on the same basis.